All I know is I've owned 3 of these vans.. 1998 Pontiac Transport with the 3.4L v6, 2002 Oldsmobile Silhouette with the 3.5L v6 and now a 2008 Chevy Uplander with the 3.9L v6 and when all 3 vans are warmed up the temperature gauge rests at the halfway mark and never moves and I live in Canada where it gets to -40c in the winter and upto +35c in the summer.
Now I've heard intake manifold and/or head gaskets have been known to cause this type of issue !
Things to check..
**Internet !
**Oil filler cap for cream colored sludge.
**Excessive white exhaust.
**Coolant in oil, oil in coolant.
**Wet spots on upper part of engine.
**Watch your temperature gauge on cold start to see when thermostat opens..
As temperature rises when engine is warming up you will see it drop back a little bit when it opens, as for example it is the only time I ever seen the temperature go past halfway mark on all three vans I've had. It rises slightly past the halfway mark as pressure builds to open the thermostat then drops back to the halfway mark.
Good luck